1. Field of the Invention
The present invention relates to an image forming apparatus for transferring an image onto a recording sheet.
2. Related Background Art
Hitherto, a color recording apparatus by an electrophotographing process using a transfer drum, a transfer sheet, and an intermediate transfer medium is known as an image recording apparatus which can multi-copy transfer. There is, further, an image recording apparatus which multiple records onto a photosensitive drum or photosensitive belt, or the like. As an example of such an apparatus, there is a color copying apparatus or a color printer as shown in FIG. 15.
In such a color image recording apparatus, developers 65 to 68 containing color developing agents (magenta, cyan, yellow, black) necessary for color recording are arranged around a photosensitive drum 72.
Latent images of the recording colors which were sequentially formed onto the photosensitive drum 72 by a laser beam 60 are developed by the developers corresponding to the respective colors and the developing agents on the photosensitive drum are transferred onto a recording paper wrapped around a transfer drum 64 by a transfer charging device 70. After the latent image of one color on the drum was developed and transferred, the developer is exchanged, the latent image of the next color component is developed by the other color developing agent and is transferred so as to be overlapped onto the preceding transfer image formed on the recording paper on the transfer drum. As mentioned above, the latent images of four colors are sequentially developed by the developing agents of the corresponding colors and are transferred so as to be overlapped onto the recording paper on the transfer drum, thereby forming a color recording image. The developing agents are thermally fixed to the recording paper by a fixing device 73.
In this instance, after that the latent image of the previous color component was developed by the corresponding developer, the developers are exchanged for a period of time until the head of the latent image of the next color component arrives at a developing position of the corresponding developer.
As shown in FIG. 16, an interval between the rear edge of the previous latent image and the front edge of the next latent image corresponds to a length L.sub.t1 of a portion 27 in which a recording paper 63 is not adhered on the transfer drum. In FIG. 15, when the using order of the developers is set to the order opposite to the rotating direction of the photosensitive drum in a manner such as 68, 67, 66, and 65, a margin of a distance to be used for exchanging the developers is equal to L.sub.t1 -y. In this instance, (y) denotes an arrangement interval among the developers.
When rotational peripheral speeds of the photosensitive drum 72 and transfer drum 64 are set to (z), a time that can be used for exchanging the developers is equal to (L.sub.t1 -y)/z,
As another image recording apparatus, there is an apparatus such that two recording papers 63 are adhered onto the transfer drum at equal intervals as shown in FIG. 17. According to the apparatus, the same latent image is successively formed on the photosensitive drum and those two latent images are developed by the same developer in a lump and are transferred onto different recording papers. Thus, although one color recording image is outputted by four rotations of the transfer drum in the apparatus of FIG. 15, according to the apparatus of FIG. 17, two recording images can be outputted by four rotations of the transfer drum, so that an output speed of the recording image is raised to the value that is twice as high as that of the former apparatus. In such latter apparatus, a time that can be used for exchanging the developers is also equal to (L.sub.t2 -y)/z by using an interval L.sub.t2 between the papers.
As further another image recording apparatus which can multi-copy transfer, there is also an apparatus such that a plurality of papers are adhered onto the transfer drum as shown in FIG. 17, only one predetermined developer is used, and the originals are exchanged by using an automatic document feeder. In case of such an apparatus, after images were recorded onto a plurality of papers by using an image of a certain original, the original is exchanged at the paper interval (L.sub.t2) and a next original image is recorded so as to be overlapped onto the same recording paper. An original exchanging time in such a case is equal to (L.sub.t2 -y)/z in a manner similar to the above-mentioned case.
In the above conventional apparatus, when images are recorded onto a plurality of recording papers at a further high speed, the peripheral speed (z) of the photosensitive drum and transfer drum has to be raised or the paper interval L.sub.t2 has to be decreased.
In case of realizing a high speed of the image recording as mentioned above, however, the time that can be used for a change in mechanical recording conditions in the multiple recording such as exchange of the developers or originals or the like decreases. There is an upper limitation of the operating speed in the change of such mechanical recording conditions and there is also an upper limitation when realizing a high recording speed.